Helicopters are known comprising a fuselage; a main rotor fitted to a top centre portion of the fuselage; and an anti-torque tail rotor for counteracting the torque transmitted from the main rotor to the fuselage.
Anti-torque tail rotors substantially comprise:                a drive shaft which rotates about a first axis;        a hub connected operatively to the drive shaft so as to also rotate about the first axis; and        a number of blades which rotate integrally with the hub about the first axis, project from the hub, and extend along respective second axes crosswise to the first axis.        
The blades are thus rotated about the first axis integrally with the drive shaft and the hub.
The blades are also fitted to the hub to rotate about their respective second axes to adjust their pitch angles with respect to the airflow, and so adjust the lift generated by the blades and hence the thrust generated by the anti-torque rotor.
In one known solution, the helicopter comprises:                a pilot-operated pedal inside the cockpit defined by the fuselage of the helicopter;        a rod movable along the first axis and connected operatively to the blades to rotate the blades by equal angles about the respective second axes and so adjust the lift generated by the anti-torque rotor; and        a mechanism which connects the pedal operatively to the end of the rod on the opposite side to the blades, and is traversed by mechanical pulses produced by impulsive operation of the pedal by the pilot.        
The helicopter also comprises a hydraulic actuator controlled by the mechanical pulses through the mechanism, and designed to supply the mechanical pulses, amplified in force, to the end of the rod.
The hydraulic actuator comprises:                a hydraulic servo-actuator controlled by the mechanical pulses and comprising a cylinder defining chambers filled with pressurized oil, and a piston that slides along a third axis crosswise to the first axis;        a first linkage which converts translation of the piston along the third axis to translation of the rod along the first axis; and        a second linkage which converts translation of the rod along the first axis to rotation of the blades about the respective second axes.        
More specifically, the hydraulic servo-actuator is fixed to a transmission group which is operatively connected to the rotor, and the first and second linkage are housed partly inside the fuselage and partly inside the anti-torque rotor.
Though satisfactory as regards correct adjustment of the blade pitch angles, the solution described still leaves room for improvement.
In particular, the known hydraulic actuator described comprises a large number of component parts, and is therefore complicated to produce, assemble and maintain.
This is substantially due to the mechanism being housed partly inside the fuselage and partly inside the anti-torque rotor of the helicopter.
A need is therefore felt within the industry for a helicopter anti-torque rotor comprising an actuator that is easy to produce, assemble and maintain.
A need is also felt within the industry to maximize as far as possible the precision and repeatability with which the blade pitch angles are adjusted, so as to improve control of the anti-torque rotor and manoeuvrability of the helicopter as a whole.
The actuator described also inevitably results in fouling inside the anti-torque rotor, mainly due to the hydraulic motor oil supply and the need to keep it pressurized. Managing this pressurized oil generates a need for frequent maintenance and for the disposal of that oil.
A need is therefore felt within the industry to adjust the blade pitch angles of the anti-torque rotor, while at the same time minimizing fouling by the actuator.
U.S. Pat. No. 2,387,617 and US2010/012309 disclose helicopters with anti-torque rotors equipped with fixed pitch angle blades and rotated by an electric motor.
U.S. Pat. No. 8,464,980 discloses using an electric motor to rotate a anti-torque rotor drive shaft.
US2009/0140095 discloses using an electric motor to rotate a helicopter anti-torque rotor.
US2013/0264412 discloses using an electric motor to rotate the drive shaft; and adjusting means interposed functionally between the electric motor and the anti-torque rotor, and designed to adjust the rotor blade pitch angles. The adjusting means comprise a memory unit and a computing unit.
U.S. Pat. No. 4,555,219 discloses a X-wing aircraft having only one rotor with four blades.
GB-A-2149372 discloses a helicopter comprising a first and a second coaxially mounted rotors and without any anti-torque tail rotor. The first rotor comprises a first main shaft and two first blades driven in rotation by the first main shaft. The second rotor comprises a second main shaft, which is mounted coaxially with the first main shaft and two second blades driven in rotation by the second shaft.
The helicopter further comprises, for each first and second rotor:                an electric motor;        a first additional shaft which is axially movable upon the action of the electric motor, angularly fixed and is connected by a first linkage to an end of the first blade; and        a second additional shaft which is axially movable upon the action of the electric motor, is angularly fixed and is connected by a second linkage to an end of the second blade.        
The first additional shaft, the second additional shaft and the relative main shaft are coaxially mounted.
U.S. Pat. No. 2,699,833 discloses a helicopter with a main rotor and an anti-torque tail rotor with blades having relative variable pitch angles. The helicopter further comprises a main motor for controlling the main rotor, and an auxiliary motor operated in response to the main rotor or through the pedals and adapted to adjust the pitch angles of the blades of the anti-torque tail rotor.
The auxiliary motor is arranged outside the anti-torque tail rotor.